High-purity Source Research Articles

Alpha-particle emission probabilities in the decay of 237Np

Measurements of the alpha-particle emission probabilities ( P α ) of 237Np have been done on two high-purity source materials of different origin, using a passivated implanted planar silicon detector. Two peak-fitting models were used to analyse the spectra. A substantial improvement of the accuracy of the P α data has been obtained.

MOCVD Growth and Doping of ZnSe and Related II-VI Materials

ABSTRACTRecent progress in metalorganic chemical vapor deposition (MOCVD) of wide bandgap II-VI materials, especially of ZnSe, ZnS and their alloys, is discussed with emphasis on the general principles for obtaining uniform and high quality epitaxial layers and the current major issue of impurity doping for achieving conductivity control. The surface morphology and crystalline quality can be improved by a suitable choice of source materials and by lattice-matching the epitaxial layer to the substrate. By using appropriate sources, high conductivity n-type epitaxial layers of ZnSe and ZnS doped with impurities from group HI and VII of the periodic table have been successfully grown by low temperature MOCVD. We have also grown p-type ZnSe layers with carrier concentration ranging from low 1016 to high 1017 cm−3 using Li3N as the dopant. Extensive studies are now focussed on the better p-type control. High purity source materials, appropriate p-type dopants and low temperature growth are important keys.

Vapour-deposited CdS films and their dependence on the source material

An investigation has been made of the conditions required for the formation of vapour-deposited CdS thin films with reproducible characteristics suitable for use in photovoltaic solar cells. Cathodoluminescence spectroscopy has been used to provide information on differences in composition between the deposited films and the source material from which they were produced, and particular consideration has been given to the influence of the source material structure and stoichiometry on the electrical properties of the resultant films. Under identical deposition conditions, four different high-purity sources were shown to produce films with very different electrical characteristics, but by selecting appropriate deposition conditions (substrate temperature and deposition rate) it was found that each material was capable of generating films with the required characteristics.

Superconductivity in Y-Ba-Cu-Ag-O and Y-Ba-Cu-Pt-O Systems

A STUDY OF THE Y-Ba-Cu-Ag-O SYSTEM: The specimens studied were produced using the standard ceramic technique. High purity source materials Y2O3, BaCO3CuO, and Ag2O were used. Solid state synthesis was carried out at 900 C for six hours in oxygen ambient. Pressed samples of 16 mm diameter and 2 mm thick were processed in an atmosphere of pure oxygen at 950 C for 12 hours. The transition temperature to the superconducting state was measured by the four point probe resistance technique. The ac magnetic susceptibility was determined by the method outlined in [1].

Near-threshold behavior of pair-production cross sections in a lead target.

A lengthy series of experiments has resulted in the production of a high-purity source of $^{86}\mathrm{Rb}$ which was used in the internal-source method of pair-production cross-section measurements. The 1.077-MeV \ensuremath{\gamma} ray in the decay of $^{86}\mathrm{Rb}$ was used to measure the absolute pair-production cross section ${\ensuremath{\sigma}}_{\mathrm{PP}=1.92\ifmmode\pm\else\textpm\fi{}0.11}$ mb in a target of Z=82. This is the lowest-energy measurement for a target with Z\ensuremath{\ge}32 and shows a significant departure from the theoretical value of 1.34 mb. This is 6% lower than our result published earlier because of a far more detailed correction for pair production by inner bremsstrahlung radiation.

Chemical beam epitaxy of InGaAs

In0.53Ga0.47As lattice matched to InP substrate has been grown by chemical epitaxy (CBE). Epilayers with lattice-mismatch Δa/a<1×10−3 have been reproducibly obtained with extreme uniformity in both composition and layer thickness over a large area of 3.8 cm diameter (limited by the present substrate holder size) without the need of substrate rotation during growth. Surface morphologies were routinely featureless and mirrorlike as observed by Nomarski phase contrast microscope. Comparing with high-quality In0.53Ga0.47As epilayers grown by molecular beam epitaxy (MBE), the CBE-grown In0.53Ga0.47As epilayers have similar linewidths but can have substantially higher 300-K photoluminescence intensities. The electron concentration and mobilities were found to be related to the source purity of the TMAs used. For epilayers grown with high-purity TMAs source, room-temperature electron mobility as high as 9000 cm2/V s and concentrations of ∼7×1015 cm−3 were produced. Such value represents the highest value obtained by using trimethylarsine as the arsenic source. In general, the electron mobilities were as good as those obtained from low-pressure metalorganic chemical vapor deposition.

Experimental Study of Ion Dynamic Effects in Overlapping Helium Lines at Low Densities

Abstract The line profiles of the overlapping helium lines 447.1 nm and 492.2 nm have been investigated in an improved plasma source of high purity at electron densities of 1 × 1021 m−3 and 3 × 1021 m−3 . The plasma parameters Ne, Te and T0 were determined carefully by independent diagnostics, which enables a critical comparison with existing theories (BCS II and MMM). We observed ion dynamic effects which considerably modify the line shape of the forbidden components. Our experiments extend the existing observations towards higher ion temperatures. Systematic trends observed earlier are critically discussed.

Decay of 61.9-minNp240high-spin isomer

A nuclear spectroscopic study of the high-spin ground state of $^{240}\mathrm{Np}$ (${I}^{\ensuremath{\pi}}={5}^{+}$) showed substantial differences from earlier work. The sources, prepared by the $^{238}\mathrm{U}(\ensuremath{\alpha}, pn)$ and $^{244}\mathrm{Pu}(n, p4n)$ reactions, were chemically separated and purified. A total of 34 $\ensuremath{\gamma}$ rays (139.9-1167.1 keV) were observed of which 13 are new; nine others which were reported previously are not confirmed. Absolute intensities of the $\ensuremath{\gamma}$ rays were determined with the aid of $4\ensuremath{\pi}$ beta measurements. A modified decay scheme is proposed and discussed; six levels in $^{240}\mathrm{Pu}$ with ${I}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}, {2}^{\ensuremath{-}}, {3}^{\ensuremath{-}}, \dots{}, {6}^{\ensuremath{-}}$ are assigned to a ${K}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}$ octupole vibrational band. The half-life measured with high purity sources is 61.9\ifmmode\pm\else\textpm\fi{}0.2 min.RADIOACTIVITY 61.9-min $^{240}\mathrm{Np}^{g}$ from $^{238}\mathrm{U}(\ensuremath{\alpha}, pn)$ and $^{244}\mathrm{Pu}(n, p4n)$; radiochemistry, Ge(Li), and $4\ensuremath{\pi}\ensuremath{\beta}$ detectors. Measured ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, ${T}_{\frac{1}{2}}$; deduced ${E}_{{\ensuremath{\beta}}^{\ensuremath{-}}}$, $log\mathrm{ft}$, $^{240}\mathrm{Pu}$ level assignments and decay scheme.

Spectroscopy of gamma rays fromAs78decay

Gamma rays accompanying the ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay of $^{78}\mathrm{As}$ to $^{78}\mathrm{Se}$ have been studied using a high purity source and large volume germanium detectors. Seventeen new $\ensuremath{\gamma}$ rays have been observed and eight previously reported gamma rays have been excluded. From singles and $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence data new levels are proposed at 1758.9, 2334.9, 2647.7, and 3372.7 keV, whereas the 1693-, 1721-, 2452-, 2798-, 3192-, 3528-, and 3984-keV levels proposed in earlier studies have been withdrawn from the $^{78}\mathrm{As}$ decay scheme. Deduced $B(E2)$ ratios have been compared with theoretical calculations based on an anharmonic oscillator model.RADIOACTIVITY $^{78}\mathrm{As}$ [from $^{78}\mathrm{Se}$ ($n$,$p$)]; measured ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coin; deduced: $^{78}\mathrm{Se}$ levels, $B(E2)$ ratios. Enriched $^{78}\mathrm{Se}$ target, Ge and Ge(Li) detectors.

New notation for phase noise and phase jitter

A new notation is proposed for use in the discussion and analysis of high-purity signal sources. Omissions in the previously accepted notation are rectified and a consistent set of interrelated symbols is suggested.

A high temperature high purity source for metal beam epitaxy

We report the first use of modulated beam mass spectrometry to assess the purity of metal beams generated from two types of Knudsen effusion oven used in metal beam epitaxy. One of these types contained sapphire components in contact with tantalum heater windings. In this case considerable contamination of gold and silver beams by Al 2O and aluminum was observed over the temperature range 1000–1400°C. Thermodynamic considerations support the view that these contaminants originated from contact reduction of the sapphire by the tantalum metal. The condensation of these impurities on an (001) indium phosphide surface at not greater than 40°C was detected by Auger electron spectroscopy. In the second type of oven all oxide components were replaced by pyrolytic boron nitride and the tantalum windings by molybdenum windings. Gold and silver beams generated from this oven were of high purity with no detectable oxide species or other beam contaminants. Aluminium beams contained Al 2O as a transitory beam impurity due to atmospheric oxidation of the aluminium charge prior to loading. The implications of the beam impurities from the first type of source are discussed, and the performance of the second type of source in generating epitaxial silver, aluminium and gold films on indium phosphide at growth temperatures not greater than 40°C is reviewed.